1. Field of the Invention
The present invention relates to a method and apparatus for shrink-wrapping items by thermally shrinking a film wrapping the items.
2. Description of the Prior Art
According to a prior art, a web of wrapping film which is thermally shrinkable is continuously pulled out from a reel and is led to a bag making unit, by which it is rendered tubular, and individual items are fed into the tube of the wrapping film. Then, opposed sheets of the tube of the wrapping film are joined in longitudinal seals along opposite marginal edges of the film and in transverse seals along the transverse ends of each item within the tube, thereby forming intermediate packages respectively containing the items within the thermal shrink film. The intermediate packages are fed through a shrinking tunnel, by which the film is heated and shrunk into close contact with the individual items contained therein. However, when the film of each intermediate package is sealed completely airtight, the air in the package is thermally expanded by the heat applied for shrinking the film. The pressure of this expanding air causes the film to swell against its shrinking force, making it impossible to shrink the film into a desired form in which the item is tightly wrapped.
A solution to this problem is to make very small perforations in the wrapping film at predetermined positions, to thus enable release of air pressure. When the intermediate package is heated, the air remaining therein is thermally expanded but does not swell the film, because the expanded air is discharged outside through the perforations. Accordingly, the film is thermally shrunk into close contact with the item and hence shrink-wraps it.
To make such perforations in the wrapping film a perforator is provided in the path along which the web of film pulled out from the reel is fed to the bag making unit. The perforator is disposed adjacent a guide roller which guides the film to the bag making unit. The perforator comprises a rod extending widthwise of the film, a plurality of rotary members mounted on the rod and a plurality of needles attached to each rotary member. By this structure, the perforations are made in the wrapping film at predetermined positions therealong while the film is transferred in contact with the rotary members.
The number and positions of such perforations to be made in the wrapping film, that is the number and positions of perforations to be made in each intermediate package, are determined according to the size of the intermediate package and the ratio in volume between the article contained therein and the internal space of the package. Hence it is necessary to adjust the number of perforations and their positions in the wrapping film in accordance with the size and shape of each article to be wrapped. Accordingly, the conventional shrink-wrapping apparatus calls for time-consuming and cumbersome operations such as selection or exchange of the above-mentioned rotary members and adjustment of the relative angular positions of the perforating needles between the rotary members each time the size and shape of the article to be wrapped are changed.
Moreover, the shrink package itself obtained with the conventional shrink-wrapping apparatus has the following defect due to the perforations and hence the resultant sealing imperfect.
For example, when the item to be wrapped is a plant, perishable food, or the like, it is preferable, for preserving its freshness or preventing its discoloration, that nitrogen gas, carbon dioxide, or similar inert gas is filled into the package together with the item to be wrapped. However, the conventional shrink package cannot be filled with such an inert gas. That is, the nitrogen gas or the like, even if filled into the shrink package, will flow out therefrom through the perforations, thus lessening the effect of the gas.
Besides, thermal shrink films used in the past, such as polyethylene, polypropylene, etc., do not have a very low gas permeability. Thus, nitrogen gas or similar inert gas filled into the shrink package leaks out therefrom little by little through the film, further lessening the above-mentioned effect.